In general, a grinder is produced by mixing and stirring a phenol resin, an epoxy resin, a polyester resin, a rubbery resin such as NBR or chloroprene, a polyvinyl alcohol resin, or the like as a binder, with a grinder granules and, if necessary, a filler, a foaming agent, a thickener, a coloring pigment, and the like, and then subjecting the mixture to molding and curing.
The structure of a grinder is composed of three elements: grinder granules, a binding material (a bonding agent), and vesicles (air voids). The performance and utility of a grinder are determined according to a combination of these three elements.
In case of rough (coarse) grinding (whetting), it becomes possible to make a grinder with moderate vesicles by binding grinder granules which have large granule sizes with a phenol resin, an epoxy resin, a polyester resin or the like having a strong binding power, thereby fixing them, and the resulting grinder is now widely utilized.
In case of precision grinding and buffing effect smaller in coarseness on surface, i.e. in case of the so-called finish grinding, however, it is a current status that a satisfactory grinder has not yet been offered.
In other words, a grinder is offered, which is produced by binding grinder granules which have small granule sizes with a phenol resin, an epoxy resin, a polyester resin or the like having a strong bonding power, thereby fixing them wherein vesicles are included. However, the binder of this grinder is a glass-like substance devoid of rubbery elasticity and is seriously brittle even processed to a foam body including air voids so that it easily disintegrates in the course of grinding, thus losing the function as grinder. In these systems, therefore, the granules to be used are divided more finely to make distribution of granularity narrower, allowed to disperse into the binder to form a dispersion which is then coated on paper, cloth or non-woven fabric and cured to offer a grinder.
When these grinders are used for precision grinding and buffing, however, a binder as a base material is too rigid (glass-like material) so that the diameter of the grinder granules inevitably influences directly coarseness of the surface of a material to be processed. Thus, it is difficult to conduct smooth, less concavo-convex even grinding. In these grinder series, therefore, a number of grinder granules having various diameters are prepared, and these grinders are used successively, from a coarse granule size to a fine granule size, to attain grinding, thus necessitating a long treatment time. Further, since a grinder is generally clogged up on use with ground dusts, the surface of the grinder deteriorated in grinding power caused by clogging is worked up (dressing) by whetting itself with a coarse grinder to renew the grinding surface. In case of a grinder obtained by comminuting the above grinder granules to be used finer, to make the granule distribution narrower, dispersing the granules into a binder such as a phenol resin, an epoxy resin, a polyester resin, coating the dispersion onto paper, cloth or non-woven fabric, and curing the coating, however, there is a drawback such that a dressing work cannot be made for the grinder clogged with ground dusts, thus incurring economical disadvantage. In other words, in case of a grinder coated on paper or cloth, a layer of the grinder granules is thin. If dressing is carried out for such a grinder, all of the grinder granules coated is detached to lose the function as a grinder. In case a grinder coated on non-woven fabric is subjected to a dressing work, the non-woven fabric is disintegrated into fibers so that the surface is not kept even to lose the function as a grinder.
On one hand, a grinder is proposed, which is prepared by fixing grinder granules which have small granule sizes with a rubbery binder such as chloroprene, neoprene and foaming the fixed material.
When a rubbery binder is foamed, however, it is softened (losing hardness) excessively so that swell is formed on the grinding surface (forming an irregular pattern) or grinding power is extremely weakened. Thus, the grinding work needs a considerable period of time.
In addition, weakness in fixing power of the grinder granules induces a number of defects such that the degree of consumption of grinder is significant.
A grinder series in which grinder granules are fixed with a polyvinyl alcohol resin is proposed. However, this series is weak in strength of the resin so that the degree of fixing the grinder granules is small. Moreover, the grinder of this series is weak in grinding power because it has a great hygroscopicity, and its effect tends to get influence of humidity and grinding property is unstable so that a number of defects arise, such as a long time being needed for grinding.
On the other hand, a foamed body containing a polyurethane resin as a binder (base material) is proposed to overcome these defects. The foamed body firmly fixes grinder granules and may contain air voids optionally so that it has been expected to be suitable for precision grinding. When the polyurethane resin is improved in hardness and strength and a foamed body is produced from the resin, however, this foamed body becomes brittle to lose the function as a grinder. On one hand, a foamed body containing a urethane rubbery elastomer as a base material is too soft so that drawbacks are found in that swell is formed or the foamed body lacks grinding property. Further, the urethane resin has a low softening point (140-150° C.). When the resin is applied to dry grinding use, fusion of a binder occurs due to grinding heat, thus showing a defect such that the grinding surface is stained and rapid deterioration of grinding power occurs. At present, therefore, the urethane resin is partially put into practice, limiting the use only for a wet grinding method.
Therefore, as a method of improving this heat resistance, a process wherein an urethane resin into which carbodiimido groups have been introduced (JP-A-53-107794 (“JP-A” means unexamined published Japanese patent application)) or an urethane resin into which isocyanurate groups and oxadoline structure have been introduced is used as a binder, and the like are proposed. Since these resins are such that a part of the urethane bonds in the urethane resins is replaced by these heat resisting bonds, but, the urethane resins as a whole are not regarded as modified so that the resins are still not beyond acquiring physical properties such as heat-resistance tolerating the dry grinding method. If these heat-resisting bonds are enhanced, this urethane resin will no longer possess rubbery elasticity and become extremely brittle, thus failing to obtain an elastic material possessing the desired toughness.
On the other hand, a process wherein a grinder material is obtained by using a polyurea resin as a binder, mixing the materials while air is caught up in the materials on stirring, heating the mixture and molding it under pressure (JP-A-11-291175) is proposed.
However, this grinder material is extremely small in the content of air (air voids) and the polyurea resin is an extremely hard material devoid of bending property and elasticity so that the grinder granules directly influences the surface to be ground. Thus, coarse grinding is possible with this grinder, but precision grinding or a polishing by buffing that a soft touch of a grinder onto the surface to be ground enables for the first time, cannot be attained.
At present, therefore, a process having been used heretofore, wherein solid grinder granules are dispersed into kerosene, a grinding oil or the like, and the dispersion is coated onto the surface to be ground, and then a non-woven fabric is rotated or reciprocally moved on the above-surface, is chiefly used as a practical method for precision grinding or buffering effect.
According to this process, the grinder granules are not fixed, as a matter of course, and grinding is extremely low in grinding power and necessitates a very long treating time. It is, however, a current status that this process is still reluctantly used.
In other words, needed for precision grinding and buffing effect (polishing) is a grinder having such nature as strong toughness tolerant to a severe dynamic action like grinding and rubbery elasticity in addition to heat resistance (high softening point) tolerant to grinding heat (including frictional heat) generated from the surface to be ground in the course of the dry grinding not only the wet grinding (a method for grinding while cooling with water), wherein a binder having a bonding power capable of strongly fixing grinder granules inherently having a small particle diameter is used, and air voids are optionally included to mitigate the directly-influence of the grinder granules on the surface to be ground, i.e. to enable mild contact of the grinder granules with the surface to be ground, thereby making it possible to grind articles evenly with less concavo-convex figure. In the current status, however, a grinder satisfying these requirements has not yet been proposed.